In a radio communication system such as WCDMA (Wideband Code Division Multiple Access), LTE (Long-Term Evolution), or LTE-A (LTE-Advanced) developed by the 3GPP (Third Generation Partnership Project) or wireless LAN or WiMAX (Worldwide Interoperability for Microwave Access) developed by the IEEE (Institute of Electrical and Electronics Engineers), a base station (a cell, a transmission station, a transmission apparatus, or an eNodeB) and a terminal (a mobile terminal, a reception station, a mobile station, a reception apparatus, or UE (user equipment)) each include a plurality of transmission and reception antennas and realize high-speed data communication by spatially multiplexing data signals using a MIMO (multi-input multi-output) technique.
When the base station transmits downlink data (transport blocks of downlink shared channels (DL-SCHs)) to the terminal in such a radio communication system, the base station multiplexes and transmits demodulation reference signals (DMRSs), which are signals known between the base station and the terminal. Here, the demodulation reference signals are also referred to as user equipment-specific reference signals (UE-specific RSs or terminal-specific (unique) RSs). The demodulation reference signals will also be referred to simply as reference signals hereinafter.
For example, the reference signals are multiplexed with the downlink data before a precoding process is applied. Therefore, the terminal can measure equalization channels including the applied precoding process and a channel state using the reference signals. That is, the terminal can demodulate the downlink data without the base station transmitting the applied precoding process.
Here, the downlink data is mapped in physical downlink shared channels (PDSCHs). That is, the reference signals are used for demodulating the PDSCHs. In addition, for example, the reference signals are transmitted only in resource blocks (also referred to as physical resource blocks or resources) in which the corresponding PDSCHs are mapped.
Here, a radio communication system has been examined that uses heterogeneous network deployment (HetNet) that includes a macro base station having wide coverage and an RRH (remote radio head) having narrower coverage than the macro base station. FIG. 11 is a schematic diagram illustrating a radio communication system that uses the heterogeneous network deployment. As illustrated in FIG. 11, for example, a heterogeneous network is constituted by a macro base station 1101, an RRH 1102, and an RRH 1103.
In FIG. 11, the macro base station 1101 constructs coverage 1105, and the RRH 1102 and the RRH 1103 construct coverage 1106 and coverage 1107, respectively. In addition, the macro base station 1101 is connected to the RRH 1102 through a line 1108 and to the RRH 1103 through a line 1109. As a result, the macro base station 1101 can transmit and receive data signals and control signals (control information) to and from the RRH 1102 and the RRH 1103. Here, as the line 1108 and the line 1109, for example, wired lines such as optical fibers or wireless lines that use a relay technique are used. At this time, when part or all of the macro base station 1101, the RRH 1102, and the RRH 1103 use the same resources, total spectral efficiency (transmission capacity) in an area of the coverage 1105 can be improved.
In addition, when a terminal 1104 is located inside the coverage 1106, the terminal 1104 can perform single-cell communication with the RRH 1102. On the other hand, when the terminal 1104 is located around an edge (cell edge) of the coverage 1106, measures against interference of the same channels from the macro base station 1101 need to be taken. Here, a method for reducing or suppressing interference with the terminal 1104 around the cell edge region has been examined in which base station cooperative communication, in which neighboring base stations cooperate with each other, is performed as multi-cell communication (cooperative communication) between the macro base station 1101 and the RRH 1102. For example, as a scheme for reducing or suppressing interference by the base station cooperative communication, a CoMP (cooperative multipoint) transmission scheme has been examined (NPL 1).